Observatory, n. A place where astronomers conjecture away the guesses of their predecessors.
Abrose Bierce, The Devil's Dictionary

Hubble's discovery of the expansion of the universe dramatically altered how humanity viewed the cosmos. The universe is a dynamic entity, evolving according to the laws of physics, and as such, can be described and understood using those laws. The existence of the CBR is a stunning confirmation that the universe began in a hot dense state known as the big bang. Within the framework of the expanding big bang model there are still may properties that must be measured through observation if we are to understand the history and future evolution of the universe. We would like to know the values of parameters such as the Hubble constant, Ho, the density parameter Omega, the age of the universe, and the geometry of the universe k. If we could measure these parameters accurately, we would be able to determine which of the many choices best describes the universe in which we live. Chapter 13 describes various tests that can, at least in principle, measure the parameters of the big bang model of cosmology.

The Hubble constant, Ho: The Hubble constant is a measure of the rate of expansion in the present universe. The subscript indicates the present value of the Hubble constant, the value we obtain by measuring the distance to relatively nearby galaxies. Of course the Hubble constant does evolve with time. As we look deeper into space we see light from galaxies at earlier and earlier epochs. Measuring red shifts and distances provides a history of the R(t), the cosmic scale factor. From this a history of the Hubble expansion rate can be derived, and with it the rate of acceleration or deceleration.

The Density parameter Omega: The matter and energy content of the universe determine its evolution. The Omega parameter is a convenient way to measure densities with respect to the critical value, namely the value that corresponds to the flat geometry and lies between the open and closed models.

The geometry of the space: Is the universe spherical, hyperbolic or flat?

The age of the universe, t o: How much time has passed since the big bang?

Astronomers have been attempting to determine these properties for over a half a century. Until recently the values were not very well known because of the difficulty of the required observations. In recent times, however, there have been great improvements in detector technology, telescope capabilities and space-based satellites.

The Hubble Space Telescope, for example, is now providing exciting new data that has helped to answer some cosmological questions. Since the telescope orbits the Earth above the obscuring atmosphere it has greater resolving power than ground based telescopes. (Here is a very beautiful picture of the HST in orbit taken during a servicing mission.) The HST was able to observe cepheid variables in the Virgo cluster of galaxies, which provided a valuable calibration for other distance measures that could be used to ever more remote galaxies. From this we now have an improved value of Hubble's constant equal to about 72 km/sec/Mpc. The Hubble results allowed the calibration of Type Ia Supernovae as standard candles. These bright supernovae can be observed to very distant galaxies indeed. Observations of these supernovae provided evidence of a most remarkable thing indeed: the present universe is accelerating rather than decelerating. This implies that there must be a significant Lambda (a cosmological constant) component to the universe.

The geometry of the universe has been determined in a remarkable way: by measurement of small temperature fluctuations in the cosmic background radiation (See Chapter 14). These observations indicate that the overall geometry is flat, k=0.

Astronomers have been studying galaxies and their motions for many years in an attempt to understand the matter content of the universe. Several significant findings have emerged: first, most of the gravitating mass in the universe appears to be in a form other than baryons, that is, the ordinary protons and neutrons that make up the elements. Second, the total mass content of the universe is below the critical value. But because there is a nonzero Lambda value this does not mean that the universe has hyperbolic spatial geometry. Rather it is flat, the Lambda term dominates over the matter term, and the universe accelerates.

Thus three separate threads of research have come together to create a unified picture of the cosmos: measurements of the red shift distance relation indicate an accelerating universe, measurements of the CBR indicate a flat universe, and studies of galaxies indicate a low matter density universe. The following table summarizes the current best model: